The present invention relates to solid state electromagnetic radiation emitting devices and, more particularly, to light-emitting diodes (LEDs).
LED devices and the technology for fabricating and using them is increasingly viewed as an attractive approach for providing solid state device light emission on a scale suitable for general illumination uses. Such devices can emit light with a power efficiency greater than 50% and that accurately mimics the visible solar spectrum. As a result, the markets for general illumination and other LED uses is expected to become quite large.
LEDs that emit radiation in the blue, green and ultraviolet portions of the electromagnetic radiation spectrum can generate white light by combining them with a suitable phosphorescent material or phosphor. In use, the phosphor is optically excited by such an LED and emits over a broad range of wavelengths resulting in the appearance of white light. The phosphor is contained in a film coating either the semiconductor LED itself or in a suitable plastic encapsulating structure housing the LED.
However, LEDs presently used in such arrangements suffer substantial self heating during operation. The optical emission efficiency of direct band-to-band electron transitions, or interband transitions, leading to photon emissions occurring in the presently used GaN/InGaN quantum well and multiple quantum well devices is dramatically reduced at elevated temperatures, and large emitting area LED based lamps used for high optical power become very inefficient for temperatures over 200° C. due to difficulties with heat extraction and dissipation. The main mechanisms behind degradation of LED efficiency at elevated temperatures are thermionic emission of injected carriers out of the quantum well or multiple quantum wells before participation in the recombination process and enhancement of the non-radiative component of recombination. Therefore, additional cooling is required to be provided to maintain high efficiency operation which, in turn, increases both manufacturing and operating costs.
This obstacle could be overcome by use of materials based on zinc oxide (ZnO) in LEDs intended for large emitting area LED lamps to be employed in solid-state lighting application. The value of using such materials derives from the high excitonic binding energy of 60 meV therein which enables efficient excitonic optical transitions in ZnO to take place at elevated temperatures. The optical mechanism of transition in the ZnO active region of LEDs is annihilation of excitons. The high excitonic binding energy is expected to prevent thermal dissociation of excitons at temperatures as high as 400° C., thereby leading to the design and fabrication of solid state optical light emitters operating at fairly high temperatures without additional cooling.
Although the lack of a reliable p-type doping process for the materials involved in the formation of p-n junctions for LEDs has been a difficulty, such diodes were recently fabricated using n-type conductivity doped zinc oxide in a junction with p-type conductivity doped aluminum gallium nitride (n-ZnO/p-AlGaN) and n-ZnO/p-GaN (omitting the aluminum) in the form of a single heterostructure grown epitaxially on silicon carbide (SiC) substrates using hybrid vapor phase epitaxy combined with chemical vapor deposition. These LEDs emitted ultraviolet light at 389 and 430 nm at room temperature, respectively, and the former was shown to operate up to 500 K. Nevertheless, there is a desire to have such LEDs with higher radiative efficiency and lower internal resistance.